Clinical and morphologic featuues are described in 23 necropsy patients with endocarditis involing rigid-framed prosthetic valves: aortic in 16 patients, and mitral in 7. Comparison of observations in the 23 patients with prosthetic valve endocarditis to those in 74 patients with active infective endocarditis involving natural left-sided cardiac valves showed significant (p less than 0.05) differences in percent with ring abscess, hemodynamic consequences of the endocarditis (valve stenosis), frequency of staphylococcus as the causative organism, and percent with complete heart block or left bundle branch block.